1. Preheat the oven to 375 degrees Fahrenheit. Mix the cherries with the lemon (use enough lemon to make it tart, but not sour). Pour filling in unbaked shell.
2. In the bowl of an electric mixer, blend the oats, brown sugar, flour, butter, cinnamon, and vanilla on low speed until it forms a coarse meal.
3. Crumble the mixture evenly over the cherry pie filling and sprinkle with a bit more lemon juice, if desired. Bake for 40 minutes.